Study at least 165 patients with Hemophilia A, obtain clinical and laboratory data as prescribed by the Coordinating Center and the Policy Board, including Factor VIII inhibitor assay by the method of Kasper, et al. (as listed in RFP-NHLI-HB-75-16), send samples to the Reference Laboratory as required, forward laboratory and clinical data to the Coordinating Center, study patients who have or develop inhibitors more frequently, and send one representative to a twice-yearly meeting of the Policy Board.